1. Field of the Invention
The present invention relates to a liquid crystal panel and, more specifically, to a liquid crystal panel with substrate having an alignment film, and to a method for driving the liquid crystal panel.
2. Description of the Related Art
It is known that generally, a ferroelectric liquid crystal molecule moves in such a manner as to rotate along the lateral surface of a cone (hereinafter called the “liquid crystal cone”) when an external force such as an electric field is applied. In a liquid crystal panel constructed by sandwiching a ferroelectric liquid crystal between a pair of substrates, the ferroelectric liquid crystal is controlled by the polarity of the applied voltage so that the liquid crystal molecules lie in one of two positions on the lateral surface of the liquid crystal cone. These two stable states of the ferroelectric liquid crystal are called the first ferroelectric state and the second ferroelectric state.
FIG. 1 shows one example of the arrangement of polarizers in a ferroelectric liquid crystal panel constructed using a ferroelectric liquid crystal. A liquid crystal cell 22 with the ferroelectric liquid crystal confined therein is placed between the polarizers 11a and 11b whose polarization axes are arranged substantially at right angles to each other (crossed Nicol configuration), with either the polarization axis “a” of the polarizer 11a or the polarization axis “b” of the polarizer 11b oriented so as to coincide with the long axis direction of the ferroelectric liquid crystal molecules when the molecules are in the first or the second ferroelectric state when no voltage is applied. In the example of FIG. 1, the polarizers are arranged with the polarization axis “a” oriented so as to coincide with the long axis direction of the ferroelectric liquid crystal molecules in the second ferroelectric state.
When the polarizers are arranged as shown in FIG. 1, light is not transmitted in the second ferroelectric state, and the ferroelectric liquid crystal panel therefore produces a black display (non-transmissive state). Depending on the polarity of the applied voltage, the ferroelectric liquid crystal is switched to the first ferroelectric state, causing the ferroelectric liquid crystal molecules to tilt at a certain angle relative to the polarization axis, so that light from a backlight is transmitted therethrough and a white display is thus produced (transmissive state). In the illustrated example, the polarizers are arranged with the polarization axis “a” oriented so as to coincide with the long axis direction of the liquid crystal molecules in the second ferroelectric state but, alternatively, the polarizers may be arranged so that the direction of the polarization axis “a” coincides with the long axis direction of the liquid crystal molecules in the first ferroelectric state. In that case, the display appears black (non-transmissive state) in the first ferroelectric state, and white (transmissive state) in the second ferroelectric state. Either arrangement can be employed in the present invention but, the following description is given by taking as an example the case where the arrangement shown FIG. 1 is employed.
FIG. 2 shows the relationship between the value of the voltage applied to the ferroelectric liquid crystal panel and the light transmittance of the ferroelectric liquid crystal panel. As shown in FIG. 2, when a positive voltage equal to or greater in magnitude than a certain value is applied to the ferroelectric liquid crystal, the ferroelectric liquid crystal exhibits the first ferroelectric state, allowing light to transmit through the ferroelectric liquid crystal panel when the polarizers are arranged as shown in FIG. 1. Conversely, when a negative voltage equal to or greater in magnitude than a certain value is applied, the ferroelectric liquid crystal exhibits the second ferroelectric state, the state in which no light is transmitted. As can be seen from the figure, the light transmittance of the ferroelectric liquid crystal is maintained even when the applied voltage becomes 0 V. That is, the display state, once written, is retained even after the applied voltage is removed.
FIG. 3 shows a typical driving method for the ferroelectric liquid crystal panel having the polarizer arrangement shown in FIG. 1. FIG. 3D shows how the amount of light (light transmittance) transmitted through one pixel in the ferroelectric liquid crystal panel varies with the applied voltage. The period ON (W) corresponds to the state that allows light transmission, and thus the pixel is in a white display state. The period OFF (B) corresponds to the state that blocks light transmission, and thus the pixel is in a black display state. Scanning electrodes and signal electrodes are formed on the opposing substrates of the liquid crystal panel, and the pixel located at each intersection of the scanning electrodes and signal electrodes is driven in the white display state or black display state. A composite voltage waveform (FIG. 3C) representing the composition of the scanning voltage waveform 3A applied to a scanning electrode and the signal voltage waveform (FIG. 3B) applied to a signal electrode is applied to the corresponding pixel in the ferroelectric liquid crystal panel.
The driving waveform shown in FIG. 3A has at least one scanning period in order to produce a display based on the first display data, and the scanning period includes a selection period (Se) for selecting the display state based on the display data and a non-selection period (NSe) for holding the selected display state; here, for writing the next display data, a reset period (Rs) for resetting, irrespective of the previously display state, the ferroelectric liquid crystal to one of the ferroelectric states is provided preceding the selection period. In the driving method illustrated by the driving waveform shown in FIG. 3, the ferroelectric liquid crystal is first reset to the first ferroelectric state for a white display state (transmissive state) in the first half of the reset period, and then to the second ferroelectric state for a black display state (non-transmissive state) in the second half of the reset period. In this way, in a ferroelectric liquid crystal display driving method, in order to produce a good display it is generally practiced to provide a reset period for switching the ferroelectric liquid crystal between the first and second ferroelectric states, irrespective of the immediately preceding display state, by applying pulses of opposite polarities.
As a grayscale display method for a ferroelectric liquid crystal panel having only two states, i.e., the first ferroelectric state and the second ferroelectric state, it is practiced to provide a voltage gradient within the same pixel and thus provide different threshold voltages within the same pixel, or to split each one pixel into a plurality of segments and apply a designated voltage to each individual segment, achieving a grayscale display based on the area ratio between the white and black display states within the pixel divided into the plurality of segments.
However, if each one pixel is divided into a plurality of segments, the number of electrodes increases, increasing the complexity of the cell structure; furthermore, since the number of ICs also increases, the cost of the panel rises. On the other hand, two main methods are used to provide a voltage gradient within the same pixel, one being by providing a voltage gradient across an electrode itself and the other by providing numerous recesses and protrusions on the electrode surface, but in either method, it is technically extremely difficult to provide the voltage gradient with good reproducibility.